1. Field of the Invention
The present invention relates to a method of manufacturing aggregate particles and a toner.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic system, toner images are formed by supplying a toner which is in a charged state, to electrostatic latent images formed on the surface of a photoreceptor, and developing the electrostatic latent images and fixing the toner images to a recording medium. In the electrophotographic system, it is demanded for forming images at high image density and excellent in image quality by uniformly depositing the toner to the electrostatic latent images. For forming images of high image density and excellent image quality, it is important that the particle size of the toner is uniform, the width of the particle size distribution is narrow, and the charging performance is uniform. The particle size of the toner gives an effect not only on the charging performance but also on the reproduction of original images at high fineness. A toner having an appropriately small particle size, that is, a toner with a particle size of about 3 to 6 μm is effective for obtaining highly fine images.
Accordingly, various studies have been made so far for making the particle size of the toner uniform and small. For example, as a method of making the toner particle size uniform, a wet method such as an aggregation method or polymerization method has been known. In the method of manufacturing the toner by the aggregation method, aggregate particles as the toner are manufactured, for example, by adding a flocculant such as a bivalent or trivalent metal salt to an aqueous slurry in which fine resin particles, colorant particles, release agent particles, etc. are dispersed thereby aggregating the resin particles, the colorant particles, and the release agent particles. In the polymerization method, a toner is obtained by stirring an aqueous medium containing at least a monomer composition containing a polymerizable monomer and a colorant and a dispersion stabilizer, granulating the monomer composition to an appropriate particle size in the aqueous medium, and polymerizing the polymerizable monomer by a previously added polymerization initiator or a newly added polymerization initiator. In the method of manufacturing the toner using the aggregation method or the polymerization method as described above, since aggregate particles of a uniform particle size can be obtained, it is not necessary to conduct a classification step and the number of manufacturing steps can be decreased compared with the method of manufacturing the toner such as a pulverization method. Further, since the aggregation method and the polymerization method are useful also in view of the cost and decrease of the wastes, further proposals have been made.
In the method of manufacturing the toner using the aggregation method, a technique that defines the stirring conditions for the stirring device in the coagulation step has been proposed (refer, for example, to Japanese Unexamined Patent Publication JP-A 2001-242663). JP-A 2001-242663 discloses a method of manufacturing a toner that defines a preferred liquid surface shape of an aqueous slurry during stirring (referred to as “mixed liquid dispersion” in JP-A 2001242663) and stirs the mixed liquid dispersion such that the liquid surface shape of the mixed liquid dispersion forms a defined liquid surface.
In JP-A 2001-242663, it is defined, as a preferred liquid surface shape of the mixed liquid dispersion during stirring, that the distance Hc between the liquid surface at a central portion of a mixed liquid dispersion and the bottom at the central portion of a reaction vessel during stirring is 0.5 times or less a distance H between the horizontal surface of the mixed liquid dispersion and the bottom at the central portion of the reaction vessel during no stirring, and a distance He between the liquid surface at the end of the mixed liquid dispersion and the bottom at the central portion of the reaction vessel is 1.5 times the distance H during stirring. By stirring the mixed liquid dispersion such that the liquid surface shape of the mixed liquid dispersion forms the suitable liquid surface shape as described above, since mixing of the mixed liquid dispersion during aggregation can be made uniform, and the width of the particle size distribution of the aggregate particles can be made narrow, so that a toner as aggregate particles having a uniform particle size and small particle size can be obtained.
Further, in the method of manufacturing the toner using the polymerization method, a technique of defining the constitution of a stirring device in the granulating step is proposed (refer, for example, to Japanese Unexamined Patent Publication JP-A 2001-255697). JP-A 2001-255697 discloses a method of manufacturing a toner by the polymerization method in a stirring device in which the position for a stirring blade provided to the stirring device and the rotational speed at the top end of the stirring blade is defined. In JP-A 2001-255697, the stirring blade is present at the position with H/D of 0.1 or more where H represents the depth from the water surface to the upper end of the stirring blade and D represents a vessel diameter and the liquid dispersion is stirred under the condition at a velocity at the top end of the stirring blade of 5 m/s or less in the stirring tank having the stirring blade. By improving the stirring condition in the granulating step as described above, a toner comprising aggregate particles of small particle size whose distribution width of particle size is narrow can be obtained.
However, in the method of manufacturing the toner disclosed in JP-A 2001-242663, since the liquid surface shape is concaved into a V-shaped form, this tends to generate macro bubbles when the liquid dispersion continuously involves a gas phase in contact with the liquid dispersion, and bubbles are tended to be included in the aggregate particles. The toner comprising aggregate particles with interfusion of bubbles is poor in mechanical strength and pulverized when stirred in a developing tank to cause a fine powder. In a case where the fine powder is present in the toner, this results in a problem of causing image fogging and blanking of fixed images.
Further, also in the method of manufacturing the toner disclosed in JP-A 2001-255697, it is difficult to prevent the generation of bubbles to result in a problem that the obtained toner comprising the aggregate particles is a toner of low mechanical strength with inclusion of bubbles.